Single frequency near-IR lasers are used in chemical sensors to produce desired wavelengths at the vibrational bands of a target sample. As many chemical species have low optical absorptivity in this region, systems using sensors based on conventional laser light sources are not particularly sensitive when combined with simple direct absorption approaches. System sensitivity can be improved by using elaborate and less robust spectroscopic approaches. To illustrate, the sensitivity of near-IR chemical sensors may be increased using ultra-long path astigmatic Herriott cells, resonant photo-acoustic approaches, or optical cavity-based methods, e.g. cavity ring-down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS).
Photo-acoustic spectroscopy, while capable of extreme sensitivity, frequency requires the use of complicated resonant cells. In addition, the relative intensities of spectra using photo-acoustic spectroscopy can be difficult to interpret due to the variation in coupling of the optical excitation of the surrounding gases.
Astigmatic, multi-pass cells are relatively large, alignment-sensitive, and susceptible to mirror contamination.
Optical cavity based approaches are even more susceptible to mirror contamination, because they explicitly extract their sensitivity from the ultra high reflectivity of the cavity optics (R=99.9-99.99% typically). Additionally, CRDS is extremely alignment sensitive. ICOS, and particularly “off-axis” ICOS, requires relatively large sample cells, e.g. 2″ diameter optics, which increases sample cell volume, pumping requirements, and overall system size. Even larger optics are required when extending this technique to the mid-IR range, further increasing cell volume.